Duration vs overlap ?

[grumpyvette]

Duration vs overlap ?

 

ok what your trying to do is keep the DCR as high as the fuel octane will allow and the duration and lift matching the rpm level you want to be most effective in, now lift per deg. of rotation is limited by the max feed ramp angle that the type of lifter your useing will allow without haveing the edge of the lifter contact the surface of the cam lobe as the cam rotates under the lifter and that is limited by the dia. of the cam lobes base circle to lift ratio and the dia. of the lifter or in the case of roller lifters the dia. of the roller wheel. thats why the ford guys have a slight advantage in that stock chevy lifters are 0.842" and fords are 0.874" this allows the ford cam lobes to be slightly more aggressive and thats also why those mushroom base lifters are used in racing engines in classes not allowing roller lifters. now the main idea is to get the valve open as fast as possiable and closed as fast as possiable within the limits the valve train can support in the rpm range you want max power in , since power is basically the engines torque x rpm and the rate at which it can be produced, and torque is basically cylinder pressure times piston surface area times leverage( max crank stroke) times the number of power strokes per minute your looking for max torque at the max rpm your engine can effectively flow air into the cylinders, so your looking for a ballance between the shortest possiable duration to maximize cylinder pressure that keeps the valves open long enough at that rpm level time wise for max air flow to fill those cylinders, ballanced against the least overlap that will provide efficient exhaust scavageing at that rpm level.now alot depends on the cylinder volume versus port flow numbers but in general (for racing)were talking about a rpm range in or near 6000-7500rpm and a intake duration of 245-255 for a 396-454 chevy with a 106-110 LSA and a .600-.700 lift for street use a rpm range of 1000-3000rpm drops you back to about 220-230 duration 108-112 LSA and .450-.550 lift but as I said before port flow and compression have a big effect on your choice here.ports that dont flow well may require a tighter LSA and longer durations in the cam you pick to allow greater time in which the cylinders can fill at those rpms and of course the 454 and larger engines haveing greater cylinder volume to fill tend to favor those longer durations and tighter lobe seperation angles more than the 396. also read these threads.

 

http://www.chevytalk.org/forums/Forum64/HTML/008208.html

 

http://www.chevytalk.org/forums/Forum64/HTML/008029.html

 

 

these are the valve timeing overlap ranges that are most likely to work correctly

trucks/good mileage towing 10-35 degs overlap

daily driven low rpm performance 30-55degs overlap

hot street performance 50-75 degs overlap

oval track racing 70-95degs overlap

dragster/comp eliminator engines 90-115 degs overlap

 

but all engines will need the correct matching dcr for those overlap figures to correctly scavage the cylinders in the rpm ranges that apply to each engines use range.

 

example

http://dab7.cranecams.com/SpecCard/DisplayCatalogCard.asp?PN=114681&B1=Display+Card

 

here is a hot street cam that works great in many CARBERATED 383 camaros with at least 10.5 static cpr with 3.5-4.1 gears

now the timeing is intake opens 29.0 btdc, closes 71.0 abdc exhaust opens 77.0 bbdc, closes 31.0 atdc so if we add the 29 to the 31 we get the overlap duration of 60 degs of which makes this cam fall in the center of HOT STREET

 

look here and copy this first,

 

http://www.crower.com/misc/valve_timing_chart.html

copy ALL the charts this gives you a referance for the intake and exhaust closeing points at .050 lift to work from on cams listed with no .050 lift specs .

while not strictly 100% accurate adding 20 degrees to both the intake BTDC and exhaust ATDC.050 timeing figure (for solid lifter cams)(HYDROLIC CAMS have the seat specs listed at .004 seat already in most cases)will get you very close to the seat to seat figures for calc purposes.

 

and the lift figures on the crane cams spec card,that state lift is figured at (0) lash means that the lash needs to be subtracted from the total lift figure just like every other manufactures solid lift cam figures, but don,t forget while solid lift cam figures are minus lash to get true valve lift hydrolic cams can loose up to .040 thousands due to lifter pushrod seat collapsing durring valve lift from valve spring pressure forceing the oil in the lifter seat out.

 

those of you that doubt it It depends on the type hydrolic lifter and the rpm range and from what Ive seen, tested, and read the RHOADS hydrolic lifters give up .040 lift at idle and , the CRANE FAST BLEED DOWN #99377 loose about .020-.030 lift at idle,(look it up in the CRANE catalog in their description of those 99377 high intensity/fast bleed lifters) but also keep in mind the hydrolics tend to gain lift as the rpms increase due to the bleed down time becomeing ever shorter as the rpms increase untill at about 3500rpm(again it depends on the type of lifter,spring pressure,oil pressure, engine clearances ,ETC.)the hydrolics tend to get close to their rated lifts.also look here ( btw they don,t work as well as the advertiseing would lead you to beleive

http://www.amotion.com/tech/rhoads.html

notice even in the RHOADS description they say about lift on a .450 lift cam (it can be more)just throw in heavy valve springs and a high lift cam and a 1.6 ratio rocker and that same .025 will be way closer to .040

Rhoads

At Idle .425"

At 2000 rpm .447"

At 3500 rpm .450"

[blueovalz]

Why is it said that the same cam for a BB is more "radical" if placed in a SB. I would think that regardless of the displacement, the timing vs piston position being identical for both motors would create identical curves for both motors (obviously, the SB curve would be under the BB curve though).

[Guest the_dj]

Hey Grumpy can you repost those links? They didn't work here. I'm really interested in learning more about engines, and I'm going to pick up those 5 books you listed before, plus I'm trying to read all the informative posts you make. Thanks for being a great source of ideas and info here

 

DJSS

[danj]

Same here. The links didn't connect. Check out the prices of those books at Amazon/Borders. I checked there and at the local Borders and the books were about 20-25% cheaper on-line and Amazon delivered them in 3-4 days. Good reading.

[grumpyvette]

OK LINKS REPOSTED AND NEW STUFF ADDED! in the POST ABOVE